1. Field of the Invention
This invention relates to computer systems and, more particularly, to self regulation of the output voltage of a pump power supply used for a flash memory cell pair reference circuit in generating reference voltages for reading, programming, and erasing flash electrically-erasable programmable read only memory (flash EEPROM) arrays.
2. History of the Prior Art
There has been a recent trend to reducing the power requirements of portable computers. In order to lower power consumption, much of the integrated circuitry used in personal computers is being redesigned to function at lower voltage levels. Some of the circuitry used in portable computers are being designed to operate at low voltage levels such as 5 volts and 3.3 volts. This helps to reduce the power needs of such computers.
Unfortunately, some devices used in portable computers require higher voltages. Flash electrically erasable programmable read only memory (flash EEPROM memory) has recently been used to store the basic input/output startup (BIOS) processes for personal computers. This memory may be erased and reprogrammed without being removed from the computer by running an update program when the BIOS processes are to be changed. However, in order to erase and reprogram flash EEPROM memory, approximately twelve volts, a voltage level not readily available from the lower voltage batteries provided in portable computers, is required.
Flash EEPROM memory array has also been used in personal computers as a type of long term random access storage. For example, a flash EEPROM memory array may be used in place of a hard disk drive is described in U.S. patent application 07/969,131, entitled "A Sector Based Storage Device Emulator Having Variable-Sized Sector", S. Wells, filed Oct. 30, 1992, and assigned to the assignee of the present invention. Such a flash memory array provides a smaller, lighter, functionally equivalent of a hard disk drive and is not as sensitive to physical damage. Such a flash memory array would be especially useful in portable computers, where space and weight are important considerations. However, these flash EEPROM memory arrays also require much higher voltages and substantially more power than that directly available from the batteries of low power portable computers.
In some electronic devices, charge pump circuits have been used to provide a high voltage from a lower voltage source. Recently, charge pumps have been integrated with flash arrays to supply voltages needed to accomplish erase and program operations of flash memory when such voltages are not available from an external source. One example of such a charge pump arrangement for providing high voltages to a flash EEPROM memory array is disclosed in detail in U.S. Pat. No. 5,414,669 entitled "Method And Apparatus For Programming And Erasing Flash EEPROM Memory Arrays Utilizing A Charge Pump Circuit", K. Tedrow et al, issued May 9, 1995, and assigned to the assignee of the present invention.
However, erasing and programming the cells of a flash EEPROM memory array requires very accurate voltages. The voltages provided by charge pumps and other circuitry implemented as part of integrated circuits, including voltage reference circuits, typically vary due to factors such as operating temperatures, supply voltages, process variations, and load currents. Moreover, when charge pumps are used to supply the voltage, the voltage level provided at the output terminal of a charge pump tends to vary substantially from a desired voltage value. Reference voltages may be used to regulate the output voltage of charge pumps. One example of a reference voltage used in a regulation circuit for a charge pump is described in U.S. Pat. No. 5,553,295 entitled "Method And Apparatus For Regulating The Output Voltage of Negative Charge Pumps", D. Pantelakis et al, issued Sep. 3, 1996, and assigned to the assignee of the present invention.
Precision voltage references are well known in prior art and there have been many circuit arrangements for providing such references. However, as applications of such voltage references have changed, so has the requirements for voltage reference circuits. For example, it is necessary to generate very high precision voltage references within the integrated circuit which contains a flash EPROM memory array or its control circuitry. Furthermore, the circuit elements needed for providing the high precision voltage reference must be produced by the same process used in manufacturing the flash EEPROM memory array. An example of a precision voltage reference circuit produced by the process used in manufacturing flash memory is disclosed in detail in U.S. Pat. No. 5,339,272 entitled "Precision Voltage Reference", K. Tedrow et al, issued Aug. 16, 1994, and assigned to the assignee of the present invention. Other requirements of the reference circuit include lower power consumption and minimal output variation. In order to increase the power supply rejection of the voltage reference circuit and decrease the supply current, it is necessary to regulate the charge pump supply for the flash pair voltage reference circuit. However, as die size is an important factor in designing and manufacturing integrated circuits, the circuitry necessary to regulate the charge pump supply needs to be minimal. It is therefore desirable to have a voltage regulation circuit that provides minimal output variation and is not affected by variations in operating temperature, supply voltage, process, or load current. Furthermore, the circuit must be produced using the same process used to produce a flash EEPROM memory array.